The present invention relates generally to via drilling systems and, more particularly, to a method and apparatus for achieving improved parameter control in a laser drilling system.
Microcircuit boards, semiconductor circuits, and a variety of other devices commonly used by the electronics and aerospace industries require multi-layered, multi-compositional materials. These materials can, for example, utilize alternating layers of metallic and non-metallic materials. Different metallic layers can be comprised of different metals (e.g., aluminum, copper, palladium, platinum, silver, nickel, or other metal or metal alloy) while different non-metallic layers can be comprised of different non-metallic materials (e.g., epoxy-bonded fiberglass, phenolic, polyimide, etc.). Utilization of such multi-layered, multi-compositional materials requires a variety of manufacturing techniques, including the ability to drill holes or vias in the material. Vias can either be blind vias (e.g., a via that only passes through a portion of the material""s layers) or through-hole vias (e.g., a via that passes through the entire material).
Although traditional drilling techniques are suitable for a variety of applications, they are typically not well suited for drilling extremely small vias in densely packed configurations. Accordingly, a lot of emphasis has been placed on developing alternate drilling techniques. One such alternate technique is laser drilling utilizing any of a variety of lasers including excimer lasers, CO2 lasers, and solid-state lasers.
Due to the requirements placed on wavelength, beam quality, pulse length, repetition rate, and output power, many applications presently use either a diode-pumped or lamp-pumped Nd:YAG, Nd:YLF, Nd:YAP, or Nd:YVO4 laser. Typically the laser is continuously pumped and laser pulses are produced by Q-switching with an electro-optic or acousto-optic modulator. To achieve the desired output wavelength, generally either the third harmonic or the fourth harmonic of the output of the laser is used.
FIG. 1 is a graph of the output power versus pulse rate for a third harmonic laser in a conventional via drilling system. As shown, the output power of the laser is highly dependent upon the repetition rate of the pump laser. As the repetition rate is increased from a few kilohertz to tens of kilohertz, the pulse energy decreases and the pulse duration lengthens. The peak power of the pulses, which is nominally proportional to the energy divided by the pulse duration, decreases dramatically at high repetition rates. Since the conversion of radiation to shorter wavelengths is driven by peak power, conversion efficiency drops off as the peak power decreases. Accordingly, a laser drilling system using such a laser typically operates at a repetition rate of approximately 1-3 kilohertz. Lasers optimized for higher repetition rates can be used, however such lasers typically have lower per pulse energies, even when operated at low repetition rates.
U.S. Pat. Nos. 5,593,606 and 5,614,114 disclose an ultraviolet laser system for use in drilling vias in multi-layered materials. The system uses the fourth harmonic of a laser based on a solid-state lasant such as Nd:YAG, Nd:YLF, Nd:YAP, or Nd:YVO4. The output beam of the disclosed laser system has a temporal pulse width shorter than about 100 nanoseconds, typically from about 40-90 nanoseconds or lower. The average power of the laser is about 100 milliwatts with a spot size diameter of between 10 and 50 microns. A repetition rate of greater than 200 hertz is disclosed with a preferred repetition rate of between 1 and 5 kilohertz. As a result of the low repetition rates, the disclosed system was only able to drill 51 micron vias at a rate of 3.9 holes per second in a 190.5 micron thick, three layer composite comprised of copper, FR4, and copper. The throughput for the same size via in a 178 micron thick composite of copper, liquid crystal polymer, and copper was 4.5 holes per second.
Although a variety of laser drilling systems have been designed, these systems typically provide the user with only a limited ability to alter critical system parameters such as repetition rate, temporal pulse width, and output power. Accordingly, what is needed in the art is a laser drilling system that allows the user the ability to alter system characteristics to match the desired application. The present invention provides such a system.
The present invention provides a laser via drilling system, and a method of use, in which critical material processing parameters such as laser pulse repetition rate, pulse duration, and fluence level are varied to meet the requirements of the material being processed. The system uses two or more independent laser systems, each of which preferably uses the third or fourth harmonic of a solid-state fundamental laser. The output beams from each of the laser systems are combined into one or more processing beams using a beam splitter. The operational flexibility of the system can be further enhanced through the use of multiple electro-optical (EO) modulators and a polarization sensitive beam splitter.
In one embodiment of the invention, the pulse repetition rate of each of two independent laser systems is maximized and set at substantially the same rate. The pulses from each laser are staggered, thereby achieving a pulse repetition rate in the combined processing beam that is twice that achievable by either of the individual lasers. Depending upon whether the individual output beams are combined into one or two processing beams, the output fluence per pulse is either equivalent to or half the fluence per pulse achievable by either of the individual lasers.
In another embodiment of the invention, an identical pulse repetition rate for each of two independent laser systems is selected. The pulse delay between the first and second laser system is selected to be comparable to the pulse width, thereby achieving long pulse duration within the processing beam without sacrificing the extraction efficiency of either of the individual lasers.
In yet another embodiment of the invention, an identical pulse repetition rate for at least two independent laser systems is selected with zero pulse delay between the systems. By combining the individual output beams into a single processing beam, the output fluence per pulse is doubled.
In yet another embodiment of the invention, an identical pulse repetition rate for each of two independent laser systems is selected with zero pulse delay between the two systems. The individual output beams are combined using a polarization sensitive beam splitter. The combined processing beam is comprised of beams of differing polarizations, thus helping to alleviate undesired asymmetric etching/cutting rates for certain materials.
In yet another embodiment of the invention, each output beam from two independent laser systems passes through an EO modulator prior to impinging upon a polarizing beam splitter. Depending upon the condition of each of the EO modulators, each of the output beams can be directed along either of two processing beam paths.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.